Best time to have another child

So I am 23 my fiancee is 24. We have 3 beautiful children ages 4, 3, and 3 months. I want one more in the future I am thinking a year and a half to two. With my first they were super close together it was hard on my body even though I was so young at the time. So I am just wondering which age gap is good. This one between my oldest two and my lo is pretty good. I can't lie. But I do not want them to be so far our from each other.